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The invention relates generally to implantable medical devices and their methods of use for stabilizing skeletal bone, and relates more particularly to implantable medical devices fabricated of nonmetals and their use for stabilizing the cervical vertebrae of a human spine.
The Stabilizer Need
In normal anatomy, the vertebrae of the cervical column are held together and to the skeleton by a complex arrangement of ligaments, tendons, and muscles. Degenerative diseases, deformities, or trauma may cause abnormal conditions. These problems generally cause or allow displacement or rotation of a vertebra relative to the adjacent vertebra. When spinal discs rupture or bulge, the intervertebral space between two adjacent vertebrae 31 and 32 can decrease and cause discomfort to the patient. Frequently the bulging does no harm, but if it compresses against the spinal cord or a nerve, it may cause pain with loss of sensation, or weakness. Torn discs, torn ligaments, spinal fractures, and other conditions that affect the vertebral joints normal function can produce spinal pain. When surgery is needed, the discs are replaced with grafts that will heal or xe2x80x9cfusexe2x80x9d with the vertebra. This implant, with its associated stabilization, maintains the vertebral position while healing takes place. This healing is referred to as xe2x80x9cspinal fusionxe2x80x9d. The objective of spinal implants is to facilitate realignment and/or fixation of spinal elements. Clinical studies have demonstrated that surgeries using spinal implants are more effective at maintaining alignment and providing rigidity to the spine than surgeries in which implants are not used. Since the introduction of stabilizers as crude plates, rods, and wires; these devices have been developed into sophisticated appliances, which can be assembled and configured to rigidize spines of any size or condition. These stabilizers provide mechanical fixation for restraint of an implanted graft material. With this fixation, displacement during healing is significantly reduced thereby reducing the failure rate.
Prior Technology
The majority of existing cervical stabilizers use plates that are bent in the axial plane to conform to the vertebrae, and along the spinal axes to maintain lordosis. Bicortical screw purchase (where the screw penetrates the near side and the far side of the vertebra) has been favored because of the increased strength of the construct and increased screw thread area within the bone. These screws are more technically challenging to place and implanting them adds an increased risk of morbidity from neural canal penetration. The reduced strength and decreased thread area of a unicortical screw purchase (where the screw penetrates only the near side of the vertebra) increases the probability of screw backout or loosening which may result in esophageal injury. Screw backout and loosening has led to the development of mechanisms for locking the screw head to the plate in unicortical screw plate designs. Such locking mechanisms not only prevent screw backout, they also reduce the tendency of the screw head to pivot within the plate. These devices contain many intricate components that increase the cost and reduce reliability of stabilizer systems. The unicortical metal devices presently available are relatively rigid devices.
Nonmetal stabilizers are preferred over metallic stabilizers because of the minimal interference with X-rays and magnetic resonant imaging (MRI) techniques used for postoperative evaluation. Bendability or precurvature of the plate is also desired to accommodate or restore the natural lordosis of the cervical spine. These, and other desirable features and advantages, are provided by the present invention, particular embodiments of which are described in the Detailed Description Of The Patent section of the present patent.
Once complete fusion has taken place the plate is no longer needed. Indeed it is undesirable because it may interfere with esophageal action or may later fracture resulting in esophageal injury. A fractured bone that has been fixed with a metallic stabilizer is much more likely to refracture if the stabilizer is removed or if the stabilizer breaks. Refracture may occur because the stress sharing or stress shielding, that the metal stabilizer provided during healing, has not allowed the bone to carry sufficient load to return to normal load bearing strength. The compression forces should be gradually transferred from the stabilizer to the healing bone. Bioabsorbable and biodegradable materials will reabsorbe into the bone and provide a gradual reduction of the plate and screw material after fusion. This allows temporal load shareing, promoting bony maturation and strengthening, and will eliminate possible internal injury, a second operation, refracture, and imaging artifacts.
The following patents are examples of the complications and stress raisers in effort to prevent screw backout. These stress raisers are not suitable for use in polymeric stabilization:
U.S. Pat. No. 5,578,034 to Estes discloses a bone screw with an enlarged head and an annular collar surrounding the bone screw shaft. The collar""s inner diameter shrinks in response to a change in temperature, trapping the collar between head and the threads of the bone screw.
U.S. Pat. No. 5,275,601 to Gogolewski discloses an absorbable screw where a portion of the length of the screw head has a three-dimensional structure consisting of corrugations or serrations around the outer surface of the head portion. These serrations will cause stress raisers that may create cracks during fatigue cycling and will lead to screw and plate failure.
The following patents are examples of materials which may be used in the devices of this patent:
U.S. Pat. No. 5,522,895 to Mikos discloses biodegradable and bioresorbable materials and treatments that may be used in the device of this present patent.
U.S. Pat. No. 6,269,716 to Amis discloses a tapered screw head for biodegradable medical implants. The screw head has a star shaped outer circumference with external features for rotation. In the disclosed patent the resorbable fastener tapered head is connected to a threaded shaft. The stress raisers of both the threaded portion and tapered head are in the high stressed area at the plate/bone interface. This design is successfully used in non-load bearing bones in facial and cranial surgeries. However it does not have the required strength for load bearing applications.
The following patent is an example of stabilizing systems that disclose or claim tapered screws:
U.S. Pat. No. 6,228,085 to Theken discloses metallic bone fixation system with a three-dimensionally anatomically contoured plate to fit the anterior lateral profile of the vertebrae and forming a ledge to maintain the space between two vertebrae. The system is designed for use as a metal plate and is suited for thoracic and lumbar spines. It uses setscrews and threads in a portion of the hole. It has irregular surfaces in the plate such as steps, spines or teeth to bite into a bone. The screw may have a tapered outer surface adjacent to the threaded portion to provide pullout resistance of the screw in the plate.
Polymeric Stabilizers
Polymeric stabilizers have been patented and implanted in animal spines, however none have been successful, because of material failure. Making a polymeric stabilization system that will compete with present titanium plates is a challenge. Most previous polymeric stabilization systems have been designed similar to metal plate systems. The successful utilization of these polymers requires a novel design, which will operate within the limitations of polymeric material properties. The toughest bio-compatible polymers available have a tensile strength {fraction (1/25)} that of titanium and they are 50 times more elastic than titanium. Any successful polymeric systems must be designed to operate with a minimum of stress concentrations and have the highest possible fatigue endurance limit.
Stress Concentrations
Failures in mechanical devices, including present metal spinal stabilization systems, usually initiate at sites of local stress concentration caused by geometrical or microstructure discontinuity. These stress failures are related to the type of material, the nature of the stress, the environmental conditions, and the geometry of the component at these local stresses. The local stress is raised or concentrated near the root of a notch and may be many times higher then the nominal stress, or the calculated stress of the cross section. Thread roots are especially vulnerable to high stresses for two reasons. First the groove bottom is nearly sharp, creating high stress concentrations and second the cross sectionalal area of the screw is decreased at the root, reducing the area reacting the force.
Endurance Limits
Materials can fracture at a level below the ultimate single cycle load strength, if the load is repeated a sufficient number of times. This reduced fracture strength is referred to as the endurance limit. Cyclic failures start as a point of minute local stress and progressively grows across the section until the remaining sectional area can no longer support the load and the part fractures in tension. The point of crack initiation may be as small as a scratch. Surface and internal defects such as roughness, scratches, notches, grooves, shoulders, and other abrupt changes in geometry will reduce the fatigue strength of the part.
Assuming that the average fusion patient strains the fusion five thousand times per day, and assuming that the bone will grow strong enough to support itself in 90 days, the stabilizer would require a fatigue life of 450,000 cycles. The Food and Drug Administration document xe2x80x9cGuidance for Spinal System 510(k)xe2x80x9d requires five million compression stress repetitions without failure
Locking Tapers
Locking tapers, sometimes called self locking or self-holding tapers, are tapered small angled round shanks that fit into round sockets with matching taper angles. These tapers are usually less than 5 degrees on a side. During engagement the shanks are firmly seated in the socket by an axial force such as tapping with a hammer or drawing in with a screw thread. These axial forces provide a normal force component that is sufficient to create frictional forces, which will resist relative rotation of the shank with the socket.
The present patent discloses a device and a method of implantation for stabilizing cervical vertebrae in a human spine for the purpose of temporarily fixing one vertebra with respect to other vertebrae and with respect to other parts of the spinal column. This device comprises a nonmetallic plate and bone screws fabricated from non-metals. The plate has a plurality of interference fit holes to engage the bone screws. The bone screw has a threaded portion that engages a predrilled and threaded hole in the vertebra or the graft. The bone screw also has an interference fit portion between the bone surface and the thread portion, with a diameter greater than the diameter of the hole. The bone screw maintains the plate in contact with the vertebra. The screw interference fit portion is pulled into a matching plate hole, locking the screw to the plate. The interference fit is configured to be self-locking, thus preventing the screw from backing out. The inventors have reduced this device to practice in a molded plate and machined screws fabricated from MacroPore 70:30 Poly(l-lactide-co-D,L-lactide) resorbable material. These devices have been successfully tested in accordance to FDA 510 (k) in excess of five million cycles.
An object of the present invention is to provide a method of implanting a device for fusion, fixation and/or for spinal stabilization.
Another object of the present invention is to provide a stabilizer device, which will degrade and disappear once the bones have healed.
Another object of the present invention is to provide a spinal fusion and stabilization system using harvested bone, absorbable implants and nonmetallic stabilization plates and plate attachment devices.
Another object of the present invention is to provide devices and methods for cervical spinal fusions, anterioraly, posteriorly, and/or laterally.